1. Field of the Invention
The present invention related generally to communication of digital information and more specifically to channel state information estimation employed in decoding transmitted digital information.
2. Description of Related Art
Digital information is transmitted from one place to another through a channel. A channel is a general term referring to the medium through which a signal is transmitted. The channel may be a radio transmission, local area network transmission, telephone transmission, etc. In each case, the received signal differs from the transmitted signal due to the effects of the transmission through the channel. In most digital communication systems it is desirable to have knowledge of this channel information, known as channel state information (CSI) at any instant of time in order to properly decode the transmitted information.
Typically, CSI is derived using synchronization ("sync") signals, which may be sounding signals or a set of pilot symbols sent by the transmitter. The sync signals are predetermined and stored in the receiver. During transmission, the receiver compares the stored sync signals with the received signals and estimates the channel impulse response (CIR). This may be performed by a mean squared error or a least squares error formulation.
The received signal may be viewed as a vector comprising an amplitude component and a phase angle. In many applications only the phase angle of the received signal is retained for decoding the signal, as in the case of differential detection. The motivation for retaining only the phase angles is that the receiver may be simplified. Also, with differential encoding and detection, the transmission is more resistant to fading and Doppler frequency shifts. In other situations the received signal may be hard-limited, where the amplitude is limited to a maximum amplitude value, and hence only the phase angles of the received signal are available for decoding. The transmitted signals is sent with message data interleaved in periods between the sync signals. Therefore, in the absence of known sync signals (pilot or sounding), the CSI must be estimated from previous CSI or determined from the received signal directly.
CSI may be used in a number of ways to enhance detection of the transmitted signal. Some examples are: